Pitched Roofs
Issues to Consider Issues to Consider
Design Considerations
Full Fill Between & Under Rafter Insulation
Partial Fill Between & Under Rafter Insulation
Between & Over Rafter Insulation
Over Rafter Insulation
Partial Fill Between & Under Rafter Insulation
Breathable Membrane - Horizontal Installation on Continuous Substrate
Breathable Membrane - Vertical Installation on Discontinuos Substrate
  New Build - Unventilated - Over Rafter Insulation


INTRODUCTION
This method of insulating is also suitable for existing buildings where the tiles/slates need replacing. For this application we recommend the use of Kingspan Kooltherm® K7 Pitched Roof Board (premium performance CFC/HCFC–free rigid phenolic insulation).

Figure 1

Over Rafter Insulation
Premium Performancezero ODP
PRODUCT DATA ACHIEVING U–VALUES

Kingspan Kooltherm® K7
Pitched Roof Board
Board Size 2.4 x 1.2
Insulant Thickness(mm) Refer to local distributor or Kingspan Insulation price list for current stock and non-stock sizes.
Facings Composite Foil
Core CFC/HCFC - free
rigid phenolic

U-value
(W/m².K)
Kingspan Kooltherm® K7
Pitched Roof Board
0.20
100
0.18
100
Thermal Conductivity
(l-value) - K7®     45mm 0.022 W/m.K

1 The requirement for a vapour control layer and/or under tile ventilation should be assessed to BS 5250: 1989 (1995). Vapour check plasterboard or a separate vapour control layer can be used as preferred.

2 For the purposes of these calculations the standards of workmanship has been assumed good and therefore the correction factor for air–gaps has been ignored. Calculations take account for the effect of using stainless steel fixing at 6 mm diameter, giving a cross–sectional area of 7.45 mm².

3 Whilst in theory, it is possible to install insulation over rafter to meet these U–values we would recommend that over and between rafters would provide a more practical solution (see pages 16 and 17) should an over rafter layer be required.

4 The l–value quoted is in accordance with the Harmonised European Standard BS EN 13166 (phenolic) using so called 90/90 principles. Comparison with alternative products may not be appropriate unless the same procedures have been followed.
FIXING DETAILS
  • A preservative treated stop rail, the thickness of the insulation should be fixed at eaves level.


    • Lay boards either following the rafters or across rafters. The boards should be tightly butted with staggered joints to improve racking performance.

    All joints between the boards running from eaves to ridge must occur over the rafters. There is no necessity to tape board joints.

    Boards can be held in position with counter battens (38 x 38 mm). Secured with Helifix In–Skew, Target Skewfast, Wallfast Timfix or similar fixings in accordance with manufacturers guidelines;

                Helifix Limited +44 (0) 20 8735 5222
                Target Fixings Limited +44 (0) 1344 777 189
                Wallfast Limited +44 (0) 23 9265 3330

  • Alternatively a sarking board can be overlaid and fixed as above. (See figure 2)


  • If exposed rafters are required inside the building, plasterboard can be laid over the rafters before the insulation is fixed. Alternatively Kingspan Kooltherm® K18 Insulated Dry-lining Board could be used, allowing the thickness of over–rafter insulation to be reduced.


  • The length of the fixings should be increased accordingly.


  • Where a greater thickness of insulation is required, or to reduce the roof build up height a layer of insulation can be used between the rafters.
  • To prevent a cold bridge, tightly pack flexible insulation material between the rafters and the cavity closer.


  • Boards should be cut using a sharp knife or a fine toothed saw.
Figure 2 (Alternative – Scottish Style Detail)  
Scottish Style Detail